1. Field of the Invention
The present invention relates to a method and apparatus for the coating of a running layer of material, in particular paper and/or cardboard.
2. Description of the Related Art
Patent Document EP 0 438 743 B1 describes a process which basically consists of applying two wet coatings on a moving layer of material whereby the second coating is applied while the first coating is still wet. The first coating is spread upon the outer surface of an applicator roller and then the coating material is pressed upon the layer of material as it is moving through a press nip that is formed between this applicator roller and a counter roller which presses the material layer against the applicator roller. The second layer of coating is applied via a jet chamber onto a spreader roller or a spreader blade in direct application on top of the first layer of coating. As an alternative to this combination of spreader roller and jet chamber another device is recommended which employs a scoop roller where a transfer roller scoops the wet coating substance out of a holding basin and directly applies it upon the moving material layer.
DE 39 22 535 C2 also describes a process to apply two layers of coating material, one on top of the other, on a moving layer of material while the bottom layer is still wet, and the equipment to facilitate the process. The first layer of coating is applied indirectly whereby the coating substance is applied onto the outer surface of an applicator roller by a combination of spreading element and jet chamber before it is transferred onto the layer of material as it is moving through a press nip formed in-between the applicator roller and a counter roller. The second layer of coating is subsequently applied directly onto the first layer of coating again with a combination of spreading element and jet chamber, where a spreader blade smoothes the surface of the second layer of coating.
For a number of different products these two processes and the associated machineries provide good results where a pre-coating and a final coating, that is two coatings on top of one another, are required, but other applications demand smoother coatings, especially a smooth top coating is often essential.